1. Field of the invention
The present invention relates to an exhaust gas recirculation control system for an internal combustion engine for controlling recirculation of exhaust gas.
2. Discussion of Backgound
There has been known that a part of the exhaust gas of an internal combustion engine is mixed with intake air for the engine, which is called recirculation of the exhaust gas, to reduce NO.sub.x as noxious components in the exhaust gas. In this case, an exhaust gas recirculation (hereinbelow, referred to as an EGR) rate has to be accurately controlled depending on operational conditions of the engine since the EGR rate influences performance of the engine, a fuel consumption rate, etc.
FIG. 7 is a diagram showing a conventional exhaust gas recirculation control apparatus disclosed, for instance, in Japanese Unexamined Patent Publication No. 93950/1980. In FIG. 6, a reference numeral 1 designates an engine block, a numeral 2 an intake manifold, a numeral 3 an exhaust manifold, a numeral 4 a fuel supply device disposed in the intake manifold 2, a numeral 5 a throttle valve, a numeral 6 an intake duct, a numeral 7 an air cleaner, a numeral 8 an engine speed detector, a numeral 9 a negative pressure introducing passage, a numeral 10 an intake air pressure detector for detecting a pressure in the intake manifold 2 through the negative pressure introducing passage 9, a numeral 11 an EGR passage communicating the exhaust manifold 3 with the intake manifold 2, numeral 12 an EGR controlling valve which is operated by a pressure-operable diaphragm, a numeral 13 an aperture detector of or the EGR control valve 12 for detecting the degree of opening of the control valve 12, a numeral 14 an EGR control circuit, a numeral 15 an atmospheric pressure introducing passage, and numeral 16 a controlled negative pressure producing device which receives a signal outputted from the EGR control circuit 14 to control the degree of opening of the EGR control valve 12 taking account of a negative pressure in the negative pressure introducing passage and the atmospheric pressure.
In the EGR control system having the construction as above-mentioned, the speed of the engine and the negative pressure, which indicate the operating conditions of the engine, are respectively detected by the engine speed detector 8 and the intake air pressure detector 10, and signals corresponding to the detected physical quantities are inputted in the EGR control circuit. A desired value (a desired aperture) for opening the EGR control valve 12, which is given by the operational conditions of the engine, is previously inputted in the EGR control circuit 14. The EGR control circuit 14 is adapted to compare a value for the desired aperture with a value indicating an acutually measured aperture which is inputted through the aperture detector 13, and transmits an output signal to the controlled negative pressure producing device 16 so that a comparison deviation (i.e. a deviation obtained by comparing the desired aperture) with the measured aperture is made zero. Namely, a negative pressure from the controlled negative pressure producing device 16 is adjusted on the basis of the output signal from the EGR control circuit 14, the negative pressure of the intake air and the atmospheric pressure, with the result that the degree of opening of the EGR control valve 12 is controlled, whereby the EGR rate is determined so as to make the deviation between the desired aperture and the measured aperture zero. In short, an EGR rate which is in conformity with the operation of the engine is obtained by feeding-back the aperture of the EGR control valve 12 to the EGR control circuit 14 through the output signal of the aperture detector 13.
In the conventional EGR control system, however, when the EGR control valve 12 is used for a long time, fine particles such as carbon particles contained in the exhaust gas deposit on the valve, whereby the EGR rate originally set to correspond to the movement of the aperture of the EGR control valve 12 changes to thereby reduce accuracy in control.